Naturally occurring polymers, such as chitosan, have been extensively studied as carriers for therapeutic protein and gene delivery systems. β-Lactoglobulin (β-LG) is a member of the lipocalin superfamily of transporters for small hydrophobic molecules. We examine the binding of milk β-lactoglobulin with chitosan of different sizes such as chitosan 15, 100, and 200 KD in aqueous solution at pH 5-6, using FTIR, CD, and fluorescence spectroscopic methods. Structural analysis showed that chitosan binds β-LG via both hydrophilic and hydrophobic contacts with overall binding constants of K(β-LG-ch-15) = 4.1 (±0.4) × 10(2) M(-1), K(β-LG-ch-100) = 7.2 (±0.6) × 10(4) M(-1), and K(β-LG-ch-200) = 3.9 (±0.5) × 10(3) M(-1) with the number of bound protein per chitosan (n) 0.9 for ch-15, 0.6 for ch-100, and 1.6 for ch-200. Chitosan 100 KD forms stronger complexes with β-LG than chitosans 200 and 15 KD. Polymer binding did not alter protein conformation inducing structural stabilization. Chitosan 100 is a stronger protein transporter than chitosan 15 and 200 KD.